In sheet-fed offset printing machines, a printing plate is attached on the plate cylinder by means of clamping bars or attachment devices arranged in a cylinder recess. First, the leading edge of the printing plate is clamped in the corresponding clamping bar. Then, the printing plate is placed around the outer circumference of the plate cylinder by turning the latter forward slowly. Finally, the trailing edge of the printing plate is clamped in the second clamping bar, or fixed in another way. The printing plate is then subjected to a tensile force in the circumferential direction of the plate cylinder by means of the second clamping bar or attachment device in which the trailing edge of the printing plate is clamped.
An attachment device, assigned to the trailing edge of the printing plate and disposed in an axially parallel recess of the plate cylinder is known from U.S. Pat. No. 3,107,609. The tensile force causing plate tensioning is applied by compression springs assigned to the attachment device. A plurality of attachment devices, spaced apart axially, provide for the attachment of the printing plate on the plate cylinder. A restoring device is provided in the form of a shaft with eccentrics which presses the attachment devices counter to the force of the spring elements in the direction of the trailing edge of the plate. In this pressed-forward position, the trailing edge of the plate can be fixed on the attachment devices. In this case, the attachment devices are mounted so as to be pivotable about an axis running parallel to the axis of the plate cylinder.
A clamping and tensioning device for the plate cylinder of printing machines of a comparable type is known from DE 4,128,994 A1. In this case, the attachment devices are designed as a clamping bar which is subdivided over the format width of the printing plate and consists of a tensioning rail and a clamping rail. The clamping bar is mounted so as to be pivotable about its axis. Compression springs are assigned to the individual clamping bar sections which apply the tensioning force after the printing plate has been clamped by pivoting the clamping bar sections. A restoring element provides the tension-relief of the printing plate by pivoting the clamping bar sections counter to the force of the compression springs.
A pivotable clamping bar is also known from copending U.S. application Ser. No. 08/056,293 entitled "Device for Clamping and Tensioning Printing Plates," issued on Jul. 5, 1994 as U.S. Pat. No. 5,325,778, and which is hereby incorporated by reference in the present application. The U.S. Pat. No. 5,325,778 claims priority from German Patent Application P 4 214 168.0-27. There the tensioning force for the printing plates is applied by means of individual compression springs in a manner similar to how this present invention is used. The clamping bar is subdivided several times over the format width and a compression spring is assigned to each individual clamping bar section. Adjusting screws, as described in the first above-mentioned U.S. Pat. No. 3,107,609, provide means to adjust the compression springs such that different tensions can be applied to sections of the printing plate. This correction possibility is important, especially in the case of printing plates in sheet-fed offset printing machines, since the printed paper tends to become distorted due to damping medium and different subject distribution, especially at the trailing edge of print. The register problems produced from these sources have to be compensated by tensioning the printing plates to different degrees over the format width.
However, the foregoing system operates such that the force correction through the adjusting screws requires a multiplicity of revolutions until the corresponding stretching forces act on the printing plate due to the series connection of a tensioning screw and a compression spring. When a printing plate is changed, the individual tensioning screws then have to be turned back to their original position again in order for the subsequent printing plate not to be distorted by the previously set tensioning force acting differently over the format width.
A clamping bar for the plate cylinder of printing machines is known from DE 3,519,869 C2, in which the rear clamping bar is supported fixedly on the plate cylinder by means of spaced-apart tensioning screws. In this case, the tensioning screws act in a positive-locking manner directly on the cylinder body so that different distortion of the printing plate can be achieved with a relatively small turning angle of the individual tensioning screws. In this case, too, the corresponding adjustments must be reversed again when an old printing plate has been removed. In order that the tensioning and the tension relief of the screws can be carried out as quickly as possible, this publication proposes a hand tool for moving the clamping bar in the tensioning direction. However, rapid turning of the tensioning screws by hand and actuation of the hand tool must take place simultaneously.